Human Reproduction — Gametogenesis, the Menstrual Cycle and Development
This chapter is best learnt as four linked timelines: how sperm are made, how eggs are made, the monthly hormonal cycle that readies the female tract, and what happens after fertilisation. The recurring exam trap is timing and hormones — which hormone peaks when, and which event it triggers — so it pays to fix the sequence rather than isolated facts. Two anchors carry most of the marks. First, oogenesis is arrested twice and finishes only at fertilisation, unlike the continuous factory of spermatogenesis. Second, the menstrual cycle turns on a mid-cycle LH surge that causes ovulation around day 14. Learn those two and the diagrams and counts fall into place.
The Menstrual Cycle
| Phase (days) | Ovary / uterus | Key hormone |
|---|---|---|
| Menstrual (1–5) | Uterine lining sheds — bleeding | All hormones low |
| Follicular (6–13) | Follicle grows; lining rebuilds | FSH → oestrogen rising |
| Ovulation (~14) | Egg released from the ovary | LH surge (the trigger) |
| Luteal (15–28) | Corpus luteum forms; lining maintained | Progesterone (from corpus luteum) |
🧠 The LH surge causes ovulation — pin that one hormone to that one event
If you memorise nothing else about the cycle, memorise that a sharp mid-cycle rise in LH — the LH surge — is what causes ovulation around day 14. Everything else hangs off this. FSH earlier grows the follicle, which secretes oestrogen; rising oestrogen eventually triggers the LH surge; the surge ruptures the follicle and releases the egg; the emptied follicle then becomes the corpus luteum, which pours out progesterone to maintain the uterine lining in the luteal phase. If the egg is not fertilised the corpus luteum degenerates, progesterone falls, and the lining is shed — menstruation — restarting the cycle. So the cause-and-effect chain to carry is: FSH → oestrogen → LH surge → ovulation → corpus luteum → progesterone → (no pregnancy) → menstruation. Questions that ask "which hormone is at its peak at ovulation" (LH) or "what maintains the endometrium after ovulation" (progesterone) are just asking you to read one link of this chain.Fertilisation and Development
- Fertilisation happens in the ampulla of the fallopian tube, not the uterus. Fusion of sperm and egg gives the diploid zygote; the sperm entry also triggers the egg to complete its second meiotic division.
- The zygote divides (cleavage) into a morula, then a blastocyst, which implants in the uterine wall about a week after fertilisation. The outer trophoblast helps form the placenta, the organ of exchange between mother and foetus.
- The three germ layers — ectoderm, mesoderm, endoderm — form from the blastocyst and give rise to all tissues. Human pregnancy (gestation) lasts about 9 months (~280 days); birth (parturition) is triggered by an oxytocin-driven feedback.
ℹ️ The placenta: an endocrine organ, not just a pipe
It is easy to picture the placenta as a passive junction where oxygen and nutrients pass from mother to foetus and wastes pass back. It does all that, but it is also a working endocrine gland. It secretes hormones — human chorionic gonadotropin (hCG), human placental lactogen, oestrogen, progesterone — that maintain the pregnancy, and hCG is the very hormone detected by a pregnancy test. Two consequences follow that examiners like. First, because the placenta takes over progesterone production later in pregnancy, the corpus luteum is no longer essential once the placenta is established — a shift of hormonal responsibility. Second, the maternal and foetal blood do not actually mix; they run close enough for diffusion across the placental membrane but stay separate, which is why exchange happens without the two circulations merging. Seeing the placenta as both a barrier and a gland, rather than an open connection, prevents the common misconception that mother and baby share one bloodstream.Key takeaways
- Spermatogenesis gives 4 sperm per meiosis (even division); oogenesis gives 1 egg + polar bodies (unequal division).
- Sertoli cells nourish sperm; Leydig (interstitial) cells secrete testosterone.
- Oocytes are arrested in prophase I from before birth and finish meiosis II only if a sperm enters.
- The mid-cycle LH surge causes ovulation (~day 14); FSH grows the follicle before it.
- The corpus luteum secretes progesterone to maintain the endometrium; its decline (no pregnancy) triggers menstruation.
- Fertilisation occurs in the ampulla of the fallopian tube; the blastocyst implants in the uterine wall.
- The placenta is an endocrine organ (secretes hCG etc.); maternal and foetal blood do NOT mix.
- Zygote → morula → blastocyst → implantation; three germ layers form all tissues; gestation ≈ 9 months.
Practice questions (8)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
Ovulation in a normal 28-day human menstrual cycle is triggered by:
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Answer: A — A surge in luteinising hormone (LH) around day 14
The sharp mid-cycle LH surge ruptures the mature follicle and releases the egg around day 14. Progesterone rises after ovulation to maintain the lining, not to cause the release, so mistaking it for the trigger reverses cause and effect.One difference between spermatogenesis and oogenesis is that oogenesis produces:
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Answer: A — One functional egg and polar bodies from each oogonium
Oogenesis divides unequally, so one large egg keeps almost all the cytoplasm while the rest is shed as tiny polar bodies. Spermatogenesis is the one that makes four functional gametes per meiosis, so "four eggs" borrows the sperm pattern by mistake.In humans, fertilisation normally takes place in the:
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Answer: A — Ampulla of the fallopian tube
The egg meets the sperm in the ampulla, the wider part of the fallopian tube, and the resulting blastocyst only later travels to implant in the uterus. Naming the uterus confuses the site of implantation with the site of fertilisation.The corpus luteum secretes which hormone to maintain the uterine lining after ovulation?
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Answer: A — Progesterone
After ovulation the follicle becomes the corpus luteum, which secretes progesterone to sustain the endometrium for a possible pregnancy. FSH and LH are pituitary hormones acting earlier in the cycle, so they do not perform this maintenance role.Testosterone in the human testis is secreted by the:
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Answer: A — Leydig (interstitial) cells
Leydig cells in the interstitial spaces of the testis produce testosterone. Sertoli cells instead nourish the developing sperm, so assigning the hormone to them confuses the two cell types the question is separating.The hormone detected by most pregnancy tests, secreted by the developing placenta, is:
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Answer: A — Human chorionic gonadotropin (hCG)
The placenta secretes hCG early in pregnancy, and its appearance in urine is what a pregnancy test detects. Adrenaline, thyroxine and insulin come from other glands and have nothing to do with confirming pregnancy.The structure that implants in the uterine wall about a week after fertilisation is the:
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Answer: A — Blastocyst
Cleavage of the zygote produces a morula and then a blastocyst, and it is the blastocyst that implants in the endometrium. The zygote is the single fertilised cell before cleavage, so it is too early a stage to implant.Which statement about the human placenta is correct?
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Answer: A — Maternal and foetal blood remain separate and do not mix
The two circulations run close enough for diffusion across the placental membrane but stay separate, so they do not mix. The placenta is also an endocrine organ secreting hCG and other hormones, which makes the "no hormones" option false as well.